Electromagnetic induction occurs when an electrical wire passes through a changing magnetic field. Generators use electromagnetic induction to convert mechanical energy into electrical energy. In an AC, or alternating current, generator, the electrical current periodically reverses direction. With a DC, or direct current, generator, the current flows only in one direction. There are several other primary differences between AC and DC generators.
Design Differences
Both AC and DC generators produce currents via electromagnetic induction. In AC generators, the coil through which the current flows is fixed, and the magnet is moving. The magnet’s north and south poles cause the current to flow in opposite directions, producing an alternating current.With DC generators, the coil through which the current flows rotates in a fixed field. The two ends of the coil attach to a commutator: different halves of a single, rotating split ring. Metal brushes connect these split rings to an external circuit. The commutator balances the charges leaving and returning to the generator, resulting in a current that does not change direction.
Primary Uses of AC and DC Generators
AC and DC generators serve different purposes. Homes typically use AC generators to power small motors and common electrical appliances. These include vacuum cleaners, food mixers, juicers and electrical fixtures.DC generators power very large electric motors — such as those needed for subway systems. Moreover, DC generators provide a reliable and efficient energy supply that can charge banks of batteries used for mobile and off-grid uses.
Practical Considerations
Currently, the overwhelming majority of generators are the AC type, because the DC units have brushes that require periodic replacement. Although the electrical systems in many autos had DC generators before the 1960s, lower-maintenance AC units replaced DC generators. Key to this advance was the introduction of the reliable solid-state diode, an inexpensive electronic component that converts AC to DC.